.TH std::scoped_allocator_adaptor::construct 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::scoped_allocator_adaptor::construct \- std::scoped_allocator_adaptor::construct

.SH Synopsis
   Defined in header <scoped_allocator>
   template< class T, class... Args >                                 \fB(1)\fP
   void construct( T* p, Args&&... args );
   template< class T1, class T2, class... Args1, class... Args2 >

   void construct( std::pair<T1, T2>* p,
                   std::piecewise_construct_t,                        \fB(2)\fP \fI(until C++20)\fP
                   std::tuple<Args1...> x,

                   std::tuple<Args2...> y );
   template< class T1, class T2 >                                     \fB(3)\fP \fI(until C++20)\fP
   void construct( std::pair<T1, T2>* p );
   template< class T1, class T2, class U, class V >                   \fB(4)\fP \fI(until C++20)\fP
   void construct( std::pair<T1, T2>* p, U&& x, V&& y );
   template< class T1, class T2, class U, class V >                   \fB(5)\fP \fI(until C++20)\fP
   void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );
   template< class T1, class T2, class U, class V >                   \fB(6)\fP \fI(until C++20)\fP
   void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );
   template< class T1, class T2, class NonPair >                      \fB(7)\fP \fI(until C++20)\fP
   void construct( std::pair<T1, T2>* p, NonPair&& non_pair );

   Constructs an object in allocated, but not initialized storage pointed to by p using
   OuterAllocator and the provided constructor arguments. If the object is of type that
   itself uses allocators, or if it is std::pair, passes InnerAllocator down to the
   constructed object.

   First, retrieve the outermost allocator OUTERMOST by calling
   this->outer_allocator(), and then calling the outer_allocator() member function
   recursively on the result of this call until reaching an allocator that has no such
   member function.

   Define OUTERMOST_ALLOC_TRAITS(x) as
   std::allocator_traits<std::remove_reference_t<decltype(OUTERMOST(x))>>

   1) Creates an object of the given type T by means of uses-allocator construction at
   the uninitialized memory location indicated by p, using OUTERMOST as the allocator.
   After adjustment for uses-allocator convention expected by T's constructor, calls
   OUTERMOST_ALLOC_TRAITS(*this)::construct.

   This overload participates in overload resolution only if U is not a   \fI(until C++20)\fP
   specialization of std::pair.
   Equivalent to
   std::apply(

       [p, this](auto&&... newargs)
       {
           OUTERMOST_ALLOC_TRAITS(*this)::construct(
               OUTERMOST(*this), p,                                       \fI(since C++20)\fP
   std::forward<decltype(newargs)>(newargs)...);
       },
       std::uses_allocator_construction_args(
           inner_allocator(),
           std::forward<Args>(args)...
       )
   );

   2) First, if either T1 or T2 is allocator-aware, modifies the tuples x and y
   to include the appropriate inner allocator, resulting in the two new tuples
   xprime and yprime, according to the following three rules:
   2a) if T1 is not allocator-aware
   (std::uses_allocator<T1, inner_allocator_type>::value == false), then xprime
   is std::tuple<Args1&&...>(std::move(x)). (It is also required that
   std::is_constructible<T1, Args1...>::value == true).
   2b) if T1 is allocator-aware (std::uses_allocator<T1,
   inner_allocator_type>::value == true), and its constructor takes an allocator
   tag

   std::is_constructible<T1, std::allocator_arg_t,
                         inner_allocator_type&, Args1...>::value == true,
   then xprime is

   std::tuple_cat(std::tuple<std::allocator_arg_t, inner_allocator_type&>(
                       std::allocator_arg, inner_allocator()
                  ),
                  std::tuple<Args1&&...>(std::move(x)))
   2c) if T1 is allocator-aware (std::uses_allocator<T1,
   inner_allocator_type>::value == true), and its constructor takes the
   allocator as the last argument

   std::is_constructible<T1, Args1..., inner_allocator_type&>::value == true,
   then xprime is

   std::tuple_cat(std::tuple<Args1&&...>(std::move(x)),
                  std::tuple<inner_allocator_type&>(inner_allocator())).
   Same rules apply to T2 and the replacement of y with yprime.
   Once xprime and yprime are constructed, constructs the pair p in allocated
   storage by calling                                                            (until
   std::allocator_traits<O>::construct(OUTERMOST,                                C++20)
                                       p,
                                       std::piecewise_construct,
                                       std::move(xprime),
                                       std::move(yprime));
   3) Equivalent to

   construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>()),

   that is, passes the inner allocator on to the pair's member types if they
   accept them.
   4) Equivalent to
   construct(p, std::piecewise_construct,
   std::forward_as_tuple(std::forward<U>(x)),

    std::forward_as_tuple(std::forward<V>(y)))
   5) Equivalent to
   construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
                                          std::forward_as_tuple(xy.second))
   6) Equivalent to
   construct(p, std::piecewise_construct,
                std::forward_as_tuple(std::forward<U>(xy.first)),
                std::forward_as_tuple(std::forward<V>(xy.second)))
   7) This overload participates in overload resolution only if given the
   exposition-only function template

   template<class A, class B>
   void /*deduce-as-pair*/(const std::pair<A, B>&);,
   /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated
   operand.

   Equivalent to construct<T1, T2, T1, T2>(p, std::forward<NonPair>(non_pair));.

.SH Parameters

   p        - pointer to allocated, but not initialized storage
   args...  - the constructor arguments to pass to the constructor of T
   x        - the constructor arguments to pass to the constructor of T1
   y        - the constructor arguments to pass to the constructor of T2
   xy       - the pair whose two members are the constructor arguments for T1 and T2
   non_pair - non-pair argument to convert to pair for further construction

.SH Return value

   \fI(none)\fP

.SH Notes

   This function is called (through std::allocator_traits) by any allocator-aware
   object, such as std::vector, that was given a std::scoped_allocator_adaptor as the
   allocator to use. Since inner_allocator is itself an instance of
   std::scoped_allocator_adaptor, this function will also be called when the
   allocator-aware objects constructed through this function start constructing their
   own members.

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to       Behavior as published              Correct behavior
                       first overload is mistakenly used constrained to not accept
   LWG 2975 C++11      for pair construction in some     pairs
                       cases
   P0475R1  C++11      pair piecewise construction may   transformed to tuples of
                       copy the arguments                references to avoid copy
   LWG 3525 C++11      no overload could handle non-pair reconstructing overload added
                       types convertible to pair

.SH See also

   construct     constructs an object in the allocated storage
   \fB[static]\fP      \fI(function template)\fP
   construct     constructs an object in allocated storage
   \fI(until C++20)\fP \fI(public member function of std::allocator<T>)\fP
